The separation of gas components from gas mixtures by a freezeout, sublimation or combined condensation and freezing process is, of course, known and is the principle with which, for example, cryogenic (liquid-air) traps operate. The freezeout process has been used, for example, to separate xenon from nitrogen in cold traps at cryogenic temperatures, the gas mixture being passed through an annular or tubular duct which is cooled and which can be provided with longitudinal ribs extending in the direction of movement of the gas through the duct. However with conventional traps of this type, there is a tendency toward the development of frost, snow or the like in the gas as the temperature drops below the saturation temperature of the xenon in the gas mixture and some danger that the flow cross section will be plugged or reduced by the precipitate spontaneously formed in the gas.
In general, the cryogenic traps of this type can operate only for some limited length of time without significant obstruction of the gas flow and, as a consequence, the separating efficiency fluctuates fairly significantly. As a result operation of such traps is less than optimal.
Another important problem with traps which may be subject to fluctuation in operation efficiency is that it is not possible generally to establish reliably a given final concentration of the component which is to be separated from the gas mixture so that one can be certain that even with continued operation for prolonged periods the product emerging from the trap will have this concentration.